The generation of short pulses with quantum cascade lasers (QCLs) remains challenging to date due to their ultrafast gain dynamics. Here, we report on active mode-locking of mid-infrared QCLs. For the first time we show, that picosecond pulses can be generated also at room-temperature using high-performance QCL material. Mounted epi-up, the QCLs emit a train of pulses as short as a 7ps with an average power of 100mW. The nonlinear autocorrelation shows reveals the famous 8:1 ratio, which proves unambiguously that the QCL operates in the mode-locked regime. This result is further verified using the beatnote spectroscopy technique SWITS.
Water is always at risk of accidental or intentional pollution that would consist of introducing a harmful chemical into a drinking water reservoir. Fiber-optics evanescent wave sensing has been shown to be an efficient sensor scheme for direct in-water sensing. Here we demonstrate a system for the detection of chemicals dissolved in water by using quantum cascade lasers (QCLs) coupled into a silver halide fiber. The study was performed over two frequency ranges: short wavelength (i.e. 3µm and 5µm) and long wavelength (between 8µm and 10µm) and using two different types of QCL source: pulsed and continuous wave.
Quantum cascade lasers are often operated in pulsed regime for low-power applications due to the large thermal dissipation required for continuous wave operation. The typical pulse length is of the order of 100 ns with a duty cycle below 1%. Fourier transform infrared spectrometers, commonly used in the mid-infrared, typically have a spectral resolution of the order of 3 GHz and rely on the acquisition of a path-difference interferogram. As a consequence, when measuring devices operated in pulsed regime such spectrometers can only measure the spectrum averaged over several pulses.
We propose a method to determine the absolute instantaneous frequency of a pulsed laser with a precision of 10 MHz. First, the light from the laser is sent through a 30 cm long Fabry-Perot resonator under vacuum. The temporal waveform of the transmitted signal, which is measured using an HgCdTe detector, contains fringes corresponding to constructive and destructive interference occurring as function of time. This experiment allows to determine the chirp rate. The Fabry-Perot cavity is then filled with a known gas exhibiting an absorption line lying within the laser emission range, which can be used as an absolute frequency reference. By combining this measurement with the chirp rate, we obtain the instantaneous frequency of the laser as a function of time. Complex spectral behavior of pulsed DFB lasers, such as mode-hopping and dual-wavelength lasing, can also be properly identified using this technique.
Thanks to its high Kerr non-linearity and its low linear absorption, silicon is a material of choice for optical devices in the mid-infrared (from 3 to 5 microns) such as microresonators. In this wavelength range, the available optical sources such as quantum cascade lasers have a limited tunability. Tuning the refractive index of silicon can be achieved by a temperature change of the chip and has been previously demonstrated on ring resonators using integrated heaters or thermo-electric elements. We present a new method for thermo-optical tuning of silicon devices by directly using the light from a laser diode operating at 450 nm. The blue light focused on the silicon induces a local elevation of temperature and thus the refractive index locally increases. When applying this method on silicon ring resonator, the elevation of temperature leads to a decreasing free-spectral range and thus shift the resonances to lower frequencies. At 4.5 µm we measured a tuning efficiency of 200 MHz per mW of incident light. Numerical simulations of the thermo-optical effect show the locality of this tuning method, and confirm the experimental results. Finally a frequency study of the response of this method is performed and a time constant of the order of the micro-second is measured. In conclusion, we propose a fast, local, and non-invasive method for tuning silicon resonators operating in the mid-infrared that can be extended to any silicon-based device.
Following the goals of single-chip integrated dual comb spectrometers, we report on recent results on mid-infrared frequency combs. We demonstrate frequency comb operation with a bi-functional quantum cascade material, which allows the integration of lasers and detectors on one chip. With this device, we hold the power and efficiency record of QCL frequency combs. In the second part, we will present first evidence of frequency comb generation using mode-locked interband cascade lasers. With the demonstration of picosecond pulse generation in the mid-infrared, we open a new path towards battery driven sensitive high-resolution spectrometers miniaturized to chip-scale dimensions.
We present a model of carrier distribution and transport accounting for quantum localization effects in disordered semiconductor alloys. It is based on a recent mathematical theory of quantum localization which introduces a spatial function called localization landscape for carriers. These landscapes allow us to predict the localization of electron and hole quantum states, their energies, and the local densities of states. The various outputs of these landscapes can be directly implemented into a drift-diffusion model of carrier transport and into the calculation of absorption/emission transitions. This model captures the two major effects of quantum mechanics of disordered systems: the reduction of barrier height (tunneling) and lifting of energy ground states (quantum confinement), without having to solve the Schrödinger equation. Comparison with exact Schrödinger calculations in several one-dimensional structures demonstrates the excellent accuracy of the approximation provided by the landscape theory . This approach is then used to describe the absorption Urbach tail in InGaN alloy quantum wells of solar cells and LEDs. The broadening of the absorption edge for quantum wells emitting from violet to green (indium content ranging from 0% to 28%) corresponds to a typical Urbach energy of 20 meV and is closely reproduced by the 3D sub-bandgap absorption based on the localization landscape theory . This agreement demonstrates the applicability of the localization theory to compositional disorder effects in semiconductors.
 M. Filoche et al., Phys. Rev. B 95, 144204 (2017)
 M. Piccardo et al., Phys. Rev. B 95, 144205 (2017)
We discuss the unambiguous detection of Auger electrons by electron emission (EE) spectroscopy from a cesiated InGaN/GaN light-emitting diode (LED) under electrical injection. Electron emission spectra were measured as a function of the current injected in the device. The appearance of high-energy electron peaks simultaneously with the droop in LED efficiency shows that hot carriers are being generated in the active region (InGaN quantum wells) by an Auger process. A linear correlation was measured between the high energy emitted electron current and the “droop current” - the missing component of the injected current for light emission. We conclude that the droop originates from the onset of Auger processes. We compare such a direct identification of the droop mechanism with other identifications, most of them indirect and based on the many-parameter modeling of the dependence of the external quantum efficiency on the carrier injection.